Maarg Manthan · Topic 6.1

Evolution and Composition of the Atmosphere

Geography › Atmosphere and Temperature · Topic 6.1

The atmosphere is the envelope of gases held around the Earth by gravity. It evolved in three stages: the loss of the primordial hydrogen and helium, the outgassing of water vapour and gases from the hot interior, and the rise of oxygen from photosynthesis. Today it is about 78 per cent nitrogen, 21 per cent oxygen and 0.9 per cent argon, with carbon dioxide, water vapour and dust as vital minor parts.

5 min read · UPSC Prelims · Geography · Atmosphere and Temperature

At a Glance

Nitrogen 78.08 per cent
Oxygen 20.95 per cent
Argon 0.93 per cent
Carbon dioxide About 0.04 per cent, over 420 ppm
Water vapour 0 to 4 per cent, variable
Exam link UPSC Prelims, Geography: Atmosphere and Temperature

Where Does It Fit?

This is the first topic of the category on the atmosphere and temperature. The previous category covered the rocks and landforms of the solid Earth, and this one begins the study of the air around it. The next topic covers the layers of the atmosphere. Previous topic: Aeolian Landforms of Deserts. Next topic: Structure of the Atmosphere.

How Did the Atmosphere Evolve?

In short: the air we breathe is the third atmosphere of the Earth, shaped by volcanoes and by life.

The atmosphere has changed greatly since the planet formed about 4.6 billion years ago. Its evolution is described in three stages.

Loss of the primordial atmosphere: the first atmosphere was mainly hydrogen and helium. It was lost to space because of the strong solar winds from the young Sun and the high temperature of the early Earth.
Outgassing: as the Earth cooled and differentiated, gases escaped from the hot interior through volcanic eruptions, a process called degassing. This produced water vapour, carbon dioxide, methane, ammonia and nitrogen, with little or no free oxygen. As the Earth cooled further, water vapour condensed and fell as rain, which filled the basins and formed the oceans, as discussed in Volcanism – Causes and Lava Types.
Role of life: the composition was modified by photosynthesis. Cyanobacteria and later plants used carbon dioxide and released oxygen, and about 2.4 billion years ago the Great Oxidation Event raised the oxygen level. Oxygen then formed the ozone layer, which made life on land possible. Much of the carbon dioxide was locked in the oceans as limestone and in the form of fossil fuels.

What Is the Composition of the Atmosphere?

The atmosphere is a mixture of gases, water vapour and solid particles. Up to about 80 km it is mixed by turbulence and has a nearly constant composition of its permanent gases.

Gas	Percentage by volume	Role
Nitrogen (N2)	78.08	Dilutes oxygen; fixed by bacteria into nutrients for plants
Oxygen (O2)	20.95	Needed for respiration and combustion
Argon (Ar)	0.93	Inert gas
Carbon dioxide (CO2)	About 0.04	Photosynthesis; greenhouse gas
Neon, helium, methane, krypton, hydrogen, ozone	Traces	Ozone absorbs ultraviolet radiation

Gases such as nitrogen and oxygen make up 99 per cent of the dry air, and the remaining part is argon, carbon dioxide and traces of many others.

Which Components Are Variable and Why Do They Matter?

Carbon dioxide: a minor but very important gas; it is transparent to incoming sunlight but absorbs outgoing heat, so it is a greenhouse gas. Its concentration was about 280 parts per million before the Industrial Revolution and is now over 420 parts per million because of the burning of fossil fuels and deforestation, which is the main cause of global warming.
Water vapour: varies from almost zero in dry deserts to about 4 per cent in the warm humid tropics, and decreases with height and with latitude. It is the source of clouds and precipitation, and it also absorbs heat. Its role in weather is in Water Vapour, Evaporation and Condensation.
Ozone: found mainly at about 15 to 35 km, in the stratosphere, where it absorbs harmful ultraviolet radiation. At ground level it is a pollutant.
Dust and aerosols: solid particles from soil, sea salt, smoke, volcanoes and pollution. They act as hygroscopic nuclei on which water vapour condenses to form clouds, and they scatter light, causing red sunsets and haze.

How Do Humans Alter the Atmosphere?

Burning of fossil fuels, farming, industry and land clearing have raised greenhouse gases, including carbon dioxide, methane and nitrous oxide, leading to warming of the climate. Chlorofluorocarbons, used in refrigeration, destroyed ozone in the stratosphere and caused the Antarctic ozone hole. The Montreal Protocol of 1987 phased out ozone-depleting substances, and the ozone layer is slowly recovering, with the Antarctic ozone hole expected to return to the 1980 levels around the middle of the century. The Kigali Amendment of 2016 phases down hydrofluorocarbons, which warm the climate. India has ratified the Kigali Amendment and set out a plan to phase down HFCs from 2028. As an analytical point, the atmosphere is a thin layer, with half of its mass within about 5.6 km of the surface, so small changes in its composition can have large effects, as the ozone hole and global warming show. The layers of this thin envelope are described in Structure of the Atmosphere.

Why Is This Topic Important?

Composition Percentages asked in Prelims
Climate Greenhouse gases
Policy Montreal and Kigali

What Came Next?

The next topic describes the structure of the atmosphere. Next topic: Structure of the Atmosphere.

Key People and Terms

Outgassing Release of gases from the interior
Greenhouse gas Gas that traps outgoing heat
Ozone layer Ultraviolet-absorbing layer
Aerosol Fine particles in air
Hygroscopic nuclei Particles where water condenses
Great Oxidation Event Rise of oxygen about 2.4 billion years ago

Exam Corner

Points to Remember

Nitrogen 78, oxygen 21, argon 0.93 per cent.
Three stages: loss, outgassing, photosynthesis.
Ozone is mostly at 15 to 35 km.
Carbon dioxide is now over 420 ppm.
Montreal Protocol dates from 1987.

Do Not Confuse With

Ozone in the stratosphere and at the ground Protective versus pollutant
Greenhouse effect and ozone depletion Warming versus UV shield loss
Oxygen and nitrogen Life-supporting versus the most abundant

Memory Hook

Seventy-eight nitrogen, twenty-one oxygen, one argon and a pinch of carbon.

Mains Angle

Questions link composition and climate. These points add depth.

Origin Volcanoes and life
Minor gases Large effect
Policy Montreal and Kigali

A Question You May Face

An original practice question, not a past paper question.

“Describe the evolution of the Earth’s atmosphere and discuss the importance of its minor components.”

How to Answer

Stages Three stages.
Composition Major and minor gases.
Importance CO2, vapour, ozone.
Human impact Warming and ozone.

GS Relevance

Prelims: gases, percentages, ozone, greenhouse gases. GS Paper 1: atmosphere; GS Paper 3: climate change.

Frequently Asked Questions

How did the atmosphere of the Earth evolve?

The atmosphere evolved in three stages. The primordial hydrogen and helium were lost to space, then gases such as water vapour and carbon dioxide were released from the interior by volcanic outgassing, and finally photosynthesis by early life added oxygen about 2.4 billion years ago.

What is the composition of the atmosphere?

Dry air is about 78.08 per cent nitrogen, 20.95 per cent oxygen and 0.93 per cent argon, with carbon dioxide at about 0.04 per cent and traces of neon, helium, methane and ozone. Water vapour varies from zero to about 4 per cent.

Why is carbon dioxide important?

Carbon dioxide is needed for photosynthesis and is a greenhouse gas, since it lets sunlight in but absorbs outgoing heat. Its concentration has risen from about 280 parts per million before industrialisation to over 420 now, driving global warming.

What is the role of ozone in the atmosphere?

Ozone in the stratosphere, at about 15 to 35 km, absorbs most of the harmful ultraviolet radiation from the Sun, protecting life. Chlorofluorocarbons depleted it, and the Montreal Protocol of 1987 controls these gases so that the ozone layer is recovering.

What role do dust particles play in the atmosphere?

Dust and other aerosols act as hygroscopic nuclei, tiny particles on which water vapour condenses to form cloud droplets, so clouds and rain depend on them. They also scatter sunlight, which gives red sunsets and haze, and they can affect climate.

PYQ Practice — Statement Analysis

1 Nitrogen is the most abundant gas in the atmosphere.

True

It forms about 78 per cent.

2 The Earth's first atmosphere was rich in oxygen.

False

It was hydrogen and helium, later lost.

3 Ozone is found mainly in the stratosphere.